A number of offset printing plate precursors for directly producing printing plates have hitherto been proposed, and some of them have already been put into practical use. Widely employed among them is a system in which a photoreceptor comprising a conductive support having thereon a photoconductive layer mainly comprising photoconductive particles, e.g., zinc oxide, and a resin binder is subjected to ordinary electrophotographic processing to form a highly lipophilic toner image thereon and the surface of the photoreceptor is then treated with an oil-desensitizing solution, referred to as an etching solution to selectively render non-image areas hydrophilic, to thus obtain an offset printing plate.
Requirements of offset printing plate precursors for obtaining satisfactory prints include: (1) the original should be reproduced faithfully on the photoreceptor; (2) the surface of a photoreceptor has affinity for an oil-desensitizing solution so as to render non-image areas sufficiently hydrophilic, while, at the same time, having water resistance; and (3) that a photoconductive layer having an image formed thereon is not released during printing and is quite receptive to dampening water so that the non-image areas retain these hydrophilic properties sufficiently that freedom from stains even on printing a large number of prints exists.
It is known that these performance properties of the printing plate precursors are influenced by the ratio of zinc oxide to the resin binder in the photoconductive layer. For example, as the ratio of the resin binder to zinc oxide particles becomes small, oil-desensitization of the surface of the photoconductive layer is increased to reduce background stains, but, in turn, the internal cohesion of the photoconductive layer per se is weakened, this results in reduction of printing durability due to insufficient mechanical strength. On the other hand, as the proportion of the resin binder increases, printing durability is improved, but background staining tends to become conspicuous. With respect to background staining, while this is a phenomenon associated with the degree of oil-desensitization achieved, it has been elucidated that the oil-desensitization of the photoconductive layer surface depends not only on the zinc oxide/resin binder ratio in the photoconductive layer, but also greatly on the kind of the resin binder used.
Resin binders which have been conventionally known include silicone resins (see JP-B-No. 34-6670 (the term "JP-B No. " as used herein means an "examined Japanese patent publication"), styrene-butadiene resins (see JP-B No. 35-1950), alkyd resins, maleic acid resins, polyamides (see JP-B-No. 35-11219), vinyl acetate resins (see JP-B-No. 41-2425), vinyl acetate copolymer resins (see JP-B-No. 41-2426), acrylic resins (see JP-B-No. 35-11216), acrylic ester copolymer resins (see JP-B-No. 35-11219, JP-B No.36-8510, JP-B-No. 40-13946), etc. However, electrophotographic light-sensitive materials using these known resins suffer from any number of disadvantages, such as low charging characteristics of the photoconductive layer; poor quality of the reproduced image, particularly dot reproducibility or resolving power; low sensitivity to exposure; insufficient oil-desensitization attained by oil-desensitization for use as an offset master, which results in background stains on prints when used for offset printing; insufficient film strength of the light-sensitive layer, which causes release of the light-sensitive layer during offset printing, with a large number of prints not being possible; susceptibility of image quality to environmental influences at the time of electrophotographic image formation, such as high temperatures and high humidities; and the like.
Particularly for use as an offset printing plate precursor, formation of background stains due to insufficient oil-desensitization presents a serious problem. In order to solve this problem, development of binder resins for zinc oxide capable of, improving oil-desensitization has variously been investigated. Such resins include, for example, a resin having a molecular weight of from 1.8.times.10.sup.4 to 1.0.times.10.sup.5 and a glass transition point of from 10.degree. to 80.degree. C. obtained by copolymerizing a (meth)acrylate monomer and a copolymerizable monomer in the presence of fumaric acid in combination with a copolymer of a (meth)acrylate monomer and a copolymerizable monomer other than fumaric acid as disclosed in JP-B-No. 50-31011 a terpolymer containing a (meth)acrylic ester unit with a substituent having a carboxylic group at least 7 atoms distant from the ester linkage as disclosed in JP-A-No. 53-54027 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"); a tetra- or pentapolymer containing an acrylic acid unit and a hydroxylethyl (meth)acrylate unit as disclosed in JP-A-No. 54-20735 and JP-A-No. 57-202544, a terpolymer containing a (meth)acrylic ester unit having an alkyl group having from 6 to 12 carbon atoms as a substituent and a vinyl monomer containing a carboxylic acid group as disclosed in JP-A-No. 58-68046; and the like.
Nevertheless, evaluations of these resins proposed for improving oil-desensitization indicate that none of them is fully satisfactory in terms of stain resistance, printing durability, and the like.
Further, investigation of resins containing a functional group capable of forming a hydrophilic group by decomposition as binder resins has also been conducted. Such resins include, for example, a resin containing a functional group capable of forming a hydroxyl group by decomposition (see JP-A-No. 62-195684, JP-A-No. 62-210475 and JP-A-No. 62-210476); a resin containing a functional group capable of forming a carboxyl group by decomposition (see JP-A-No. 62-21269); and the like.
These resins form a hydrophilic group by hydrolysis or hydrogenolysis with an oil-desensitizing solution or dampening water used during printing. It is disclosed that use of such resins as binder resins for lithographic printing plate precursors brings about overcoming of various problems (worsening of smoothness, worsening of electrophotographic characteristics, and so on) which would be caused by the strong interaction between the hydrophilic group and the surface of the photoconductive zinc oxide grains when a resin previously having a hydrophilic group per se is used and also further elevates the hydrophilicity of the non-image areas to be made hydrophilic by the oil-desensitizing solution because of the hydrophilic group to be formed in the resins by decomposition thereof so that the lipophilicity of the image areas may be differentiated distinctly from the hydrophilicity of the non-image areas to be able to prevent a printing ink from adhering to the non-image areas during printing, whereby, as a result, a large number of prints having a sharp image quality without background staining may be obtained.
However, even such resins have been found still unsatisfactory in terms of stain resistance and printing durability. Specifically, even when the resins containing a hydrophilic group-forming functional group as mentioned above are used, if the amount of the resin to be incorporated is made large so as to further improve the hydrophilicity thereof in the non-image areas, the resin would become highly hydrophilic and thereby soluble in water because of the hydrophilic groups formed by decomposition of the resin. Accordingly, use of such excess resins has been found problematic in terms of the durability thereof.
On this basis, development of a technique for further improving the non-image areas because of the effect of the hydrophilic group to be formed the binder resin, while also improving the durability of the resin used, is desired.
More specifically, development of a technique capable of maintaining the effect of improving the hydrophilicity in the non-image areas or capable of improving the effect even when the content of the resin having a hydrophilic group-forming functional group as mentioned above in the total resin binder is lowered, so that a large number of prints having a sharp image quality without background staining may be obtained even when the printing condition has become more severe, for example, because of use of a large-scaled printing machine or because of variation in printing pressure, is desired.